1. Field of the Invention
The present invention relates to a rotary cathode X-ray tube equipment for X-ray CT which is constructed so as to permit radiation of X-ray from all directions with respect to the whole circumference of a subject.
2. Description of the Prior Art
FIG. 1 is a sectional view of a conventional rotary cathode X-ray tube equipment which is disclosed, for example, in Japanese Patent Laid Open No. 115738/83. In the same figure, a vacuum vessel 1 is installed on the floor through a support base 2. The vacuum vessel 1 is in the form of a ring provided centrally with a subject insertion hole 4 for passing a subject 3 therethrough. Its interior is maintained in high vacuum by means of a vacuum pump 5. An anode target 6 and a cathode portion 7 are disposed at a predetermined spacing within the vacuum vessel 1, the cathode portion 7 being connected to brushes 8 and 9. Between the brushes 8, 9 and the cathode portion 7 is provided a shielding plate 75 so that dust resulting from wear of the brushes 8 and 9 may not move toward the cathode portion 7, to which is applied a high voltage level of about -70 kV.
The anode target 6 is for generating X-ray under the impingement thereon of an electron beam emitted from the cathode portion 7. It is formed in a ring shape and is rotatable about a rotational axis a. More specifically, the anode target 6 is fixed to a target rotor 11 through a support member 10. The target rotor 11 is supported in a non-contact state by means of a magnet 12 which is for magnetic levitation, and is given a rotating torque by a drive means (not shown) so as to rotate around the rotational axis a. In operation, a high voltage level of about +70 kV is applied from the exterior to the target rotor 11 through a brush 13.
A touch-down bearing 14 is disposed near the target rotor 11, for supporting the target rotor 11 when the power source for the magnetic levitation magnet 12 has turned off or when it has become impossible to control the magnet 12 properly. During normal rotation of the target rotor 11, the touch-down bearing 14 does not contact the same rotor.
On the other hand, the cathode portion 7 is ring-shaped and serves as a charged particle generating mechanism. It is fixed to a cathode-portion rotor 15. The cathode-portion rotor 15 is supported in a non-contact state by means of a magnet 16 which is for magnetic levitation and which is located inside the rotor 15, and is given a rotating torque by a drive means (not shown) so as to rotate about the rotational axis a in the direction opposite to the target rotor 11. Consequently, the cathode portion 7 rotates in the direction opposite to the anode target 6. Further, an X-ray radiation window 17 is provided, and near the window 17 are disposed a collimator 18 and a ring-like detector 19 which is coaxial with the rotational axis a.
In the vicinity of the cathode-portion rotor 15 there is provided a touch-down bearing 20 which has the same function as that of the touch-down bearing 14. Between the brush 13 and the anode target 6 there is disposed a shielding plate 76 so that dust resulting from wear of the brush 13 may not move toward the anode target 6. The numerals 21 and 22 each denote a brush for placing a cathode-side rotor 23 and an anode-side rotor 24 at earth potential. Numerals 77 to 81 each denote an electrical insulator for insulation of a high voltage. Between the anode-side rotor 24 and the anode target 6 there is provided an insulating material 82, and between the cathode-side rotor 23 and the cathode portion 7 there is provided an insulating material 83.
The following description is now provided about the operation of the conventional rotary cathode X-ray tube equipment shown in FIG. 1. First, an electron beam emitted from the cathode portion 7 impinges on a focal point 25 of the anode target 6. X-ray 26 generated from the focal point 25 passes through the X-ray radiation window 17, then passes through the subject 3 and thereafter enters the ring-shaped detector 19. An output of the detector 19 is transmitted to a computer (not shown) for image reconstruction by means of a data collection circuit (not shown) and is converted to a coaxial tomography of the subject 3 in accordance with a predetermined reconstruction program.
In the conventional rotary cathode X-ray tube equipment constructed as above, since the magnetic levitation magnets 12 and 16 are disposed outside the vacuum vessel 1, the gap between each of the magnets 12, 16 and the object attracted thereby becomes large. Therefore, it is necessary for the magnets 12 and 16 to be strong. That is, the size, weight and power consumption of the rotary cathode X-ray tube equipment are increased.